Generally, a suspension device is interposed between a vehicle body and a vehicle wheel in an automobile or motorcycle or the like, and includes a shock absorber that generates a damping force and a suspension spring that biases the shock absorber in an extension direction to elastically support the vehicle body. Thereby, the suspension device can absorb impacts due to unevenness on a road surface with the suspension spring and suppress extension/compression of the suspension spring that occurs along with such impact absorption with the shock absorber, and thus the suspension device can suppress the transmission of impacts due to unevenness on a road surface to the vehicle body.
For example, the suspension device disclosed in JP2010-185572A is a front fork that suspends a front wheel in a saddle-type vehicle such as a motorcycle or a three-wheeled vehicle.
The shock absorber in the above-mentioned suspension device includes a shock absorber main body having an outer tube and an inner tube that is retractably inserted into the outer tube. The suspension spring of the above-mentioned suspension device is an air spring that is accommodated within the shock absorber main body. The air spring consists of compressed gas that is sealed within an air chamber formed within the shock absorber main body that expands/contracts in volume as the shock absorber extends/compresses so as to constantly bias the shock absorber in an extension direction.
In this way, when the suspension spring is an air spring, the weight of the suspension device can be reduced compared to a case in which the suspension spring is a coil spring. However, in an initial stage of a stroke of the suspension device in which the compression amount of the shock absorber is small, the suspension device may not be compressed quickly and this can lead to impairment in the riding comfort of the vehicle. Therefore, the above-mentioned suspension device includes a balance spring that counteracts any opposing force from the air spring (suspension spring) in the initial stage of the stroke of the suspension device, and thereby impairment in the riding comfort of the vehicle is suppressed.